Wavelength division multiplexing (WDM) apparatuses have been used when constructing a backbone network with the significant increases in the capacity of networks.
In a WDM apparatus, a redundant configuration designed to increase reliability is employed for a network (see, for example, Japanese Laid-open Patent Publication Nos. 2001-77783 and 2004-96514). In order to select a redundant line (for example, an EAST line/WEST line), a switch (SW) unit having a switching function is used in the WDM apparatus. However, for the purpose of, for example, enhancing the efficiency of the WDM apparatus in terms of the number of lines it contains, the redundant configuration is sometimes implemented by using an optical coupler for connection with a subscriber (client) device, in place of mounting the SW unit in the WDM apparatus.